Printed circuit boards, or PCBs, are generally used to mechanically support and electrically connect electronic components using conductive pathways, or traces etched from sheets of electrically conductive material (e.g., typically copper sheets) laminated onto a non-conductive substrate. A PCB populated with electronic components is referred to as a printed circuit assembly (PCA), also known as a printed circuit board assembly (PCBA). PCBs are generally rugged, inexpensive, and can be highly reliable. They require much more layout effort and higher initial cost than either wire-wrapped or point-to-point constructed circuits, but are much cheaper and faster for high-volume production. Some PCBs have trace layers inside the PCB and are called multi-layer PCBs, and may, for example, be formed by bonding together separately etched thin boards. Some multi-layer PCBs may include several layers (e.g., 4 layers, 12 layers, 24 layers, or more). Among the layers, the electrically conductive sheets are typically specified in terms of an amount of electrically conductive material (e.g., copper weight in ounces such as 0.5 oz, 1.0 oz, etc.), and such amount translates to the “thickness” of a given electrically conductive sheet or combined sheets (e.g. two 0.5 oz sheets are the same thickness as one 1.0 oz sheet).
Holes are typically drilled through a PCB with tiny drill bits (e.g., made of solid tungsten carbide) and/or LASERs in order to connect components to different layers of the PCB. The drilling may be performed by automated drilling machines, with the placement of the holes controlled by a drill tape or a computer generated drill file. The drill file describes the location and size of each hole to be drilled in the PCB. These holes are generally referred to as “vias.” These vias are often plated with conductive material (e.g., copper or aluminum) forming annular rings, which allow the electrical and thermal connection of conductors on opposite sides of a PCB.
It is also possible with controlled-depth drilling, laser drilling, or by pre-drilling the individual sheets of the PCB before lamination, to produce holes that connect only some of the copper layers, rather than passing through the entire board. These holes are called “blind vias” when they connect an internal copper layer to an outer layer, or “buried vias” when they connect two or more internal copper layers and no outer layers. The walls of the vias, for boards with 2 or more layers, are generally plated with copper to form plated-through-holes (PTHs) that electrically connect the conducting layers of the PCB.
After the printed circuit board (PCB) is completed, electronic components must be attached to the PCB to form a functional PCBA. In through-hole construction, electronic component leads, pins or the like are inserted in PTHs in the PCB. In surface-mount technology (SMT) construction, the components are placed on pads or lands on the outer surfaces of the PCB. In both kinds of construction, component leads are electrically and mechanically fixed to the PCB with molten metal solder.
PTH electronic components may be attached to a PCB using a soldering technique referred to as wave soldering. Wave soldering is a large-scale soldering process by which electronic components are soldered to a PCB to form an electronic assembly. The name is derived from the use of waves of molten solder to attach metal components to the PCB. The process uses a tank to hold a quantity of molten solder, and the components are inserted into or placed on the PCB and the loaded PCB is passed across a pumped wave or fountain of solder. The solder “wets” the exposed metallic areas of the board (e.g., those not protected with solder mask, a protective coating that prevents the solder from bridging between connections), creating a reliable mechanical and electrical connection. The process is much faster and can create a higher quality product than manual soldering of components. Wave soldering is used for both through-hole printed circuit assemblies and surface mount assemblies.
While there are many types of wave solder machines, the basic components and principles of these machines are generally the same. A standard wave solder machine includes three zones: the fluxing zone, the preheating zone, and the soldering zone. An additional fourth zone, a cleaning zone, may also be used depending on the type of flux applied.
When a PCB enters the fluxing zone, a fluxer applies flux to the underside of the board. Two types of fluxers are used: a spray fluxer and a foam fluxer. For either flux application method, precise control of flux quantities is required. Too little flux will cause poor joints, while too much flux may cause cosmetic or other problems. Also, as can be appreciated, the types of flux may affect the end result.
The PCB will then enter the preheating zone. The preheating zone consists of convection heaters, which blow hot air onto the PCB to increase its temperature. Generally, preheating is necessary to activate the flux, and to remove any flux carrier solvents. Preheating is also necessary to prevent thermal shock, which may occur when a PCB is suddenly exposed to the high temperature of the molten solder wave.
The tank of molten solder has a pattern of standing waves (or, in some cases, intermittent waves) on its surface. When the PCB is moved over this tank, the solder waves contact the bottom of the board, and stick to the solder pads and component leads by surface tension. For the pins of PTH components, molten solder fills the holes around the pins by capillary action. Precise control of wave height is required to ensure solder is applied to all areas but does not splash to the top of the board or other undesired areas. This process is sometimes performed in an inert gas nitrogen (N2) atmosphere to increase the quality of the joints.
As the thickness of a PCB increases (e.g., above 100 mils, 150 mils, 200 mils, or more) and the combined weight of the copper sheets increases (e.g., above 0.5 oz, 1.0 oz, 1.5 oz, 2.0 oz, or more), it may become more difficult to successfully fill the PTHs during the soldering process. One cause of the increased difficulty is that the molten solder tends to cool (“freeze”) prematurely before it has traveled from the bottom of the PCB to the top. The problem of premature freezing of the molten solder can be particularly acute when lead free solder is used in the soldering process. This problem can be further exaggerated in PTHs that are used for ground and power connections. The reason for this is that a multilayered PCB may include several ground or power planes (e.g., 4 layers, 8 layers, 12 layers, or more) that include large sheets of copper. The multiple layers of copper sheets may conduct heat away from the molten solder (i.e., act as heat sinks), causing the solder to freeze prematurely and causing the PTH to be only partially filled with solder (e.g., 75% filled, 50% filled, or less). When the PTH is only partially filled with solder, the mechanical and electrical integrity of the solder connection may be significantly reduced or may even be ineffective. In this regard, standards have been set to require a minimum amount of solder that fills a through hole for various components. For example, the Institute for Interconnecting and Packaging Electronic Circuits (IPC) requires solder to fill at least 75% of the through hole for a signal pin and at least 50% of the through hole for a ground or power pin.
FIGS. 1 and 2 illustrate top and cross-sectional views of a PCB 100 that includes PTH components. The PCB 100 is configured with a resistor 104 and an integrated circuit (IC) 106. The PCB 100 includes a plurality of PTHs 110A-110H that may be used to couple electronic components (e.g., the resistor 104 and the IC 106) from the top layer 102 of the PCB 100 to one or more conductors (not shown in FIG. 1) within or on the bottom surface of the PCB 100. In this regard, the PTHs 110 may receive component leads 105A-105B, 107A-107-E extending from the electronic components 104, 106. The PCB 100 may also include a plurality of metal traces (e.g., copper traces 111) that are operative to couple different components of the PCB 100 together. The component leads 105A-105B, 107A-107E may also be referred to herein as pins.
FIG. 2 illustrates a cross-sectional view of a portion of the PCB 100 shown in FIG. 1 cut at the line 2-2. As shown, the PCB 100 includes a plurality of dielectric layers 102, 120, 122, 124, 126, 128, 130, 132 and 134. The PCB 100 also includes a plurality of electrically conductive layers 114A-114D, 116A-116B, 118A-118B disposed between (or outside of) the dielectric layers (e.g., the conductive and dielectric layers alternate). In the example shown, the IC 106 is coupled to the conductive layers 114A-114-D (e.g., ground planes) of the PCB 100 by soldering a first pin 107A (e.g. the ground pin) of the IC 106 to the PCB 100 using a first PTH 110A partially filled with solder 140. The IC 106 is also coupled to the conductive layers 118A-118-B (e.g., power planes) of the PCB 100 by soldering a second pin 107B (e.g. the power pin) of the IC 106 to the PCB 100 using a second PTH 110B partially filled with solder 140. Additional pins (not shown in FIG. 2) of the IC 106 may be coupled to additional conductive layers 116A-116B (e.g. signal planes) of the PCB 100 by soldering the additional pins of the IC 106 received within additional PTHs (not shown in FIG. 2) of the PCB 100. In this regard, the PCB 100 may include signal planes, ground planes, or power planes that are connected to other components.
As shown, solder 140 is used to mechanically and electrically couple the IC 106 to the PCB 100. In this regard, the pins 107A-107B are respectively coupled via the solder 140 and the respective PTHs 110A-110B to the respective conductive layers 114A-114D, 118A-118B. It is noted that the conductive layers 116A-116B (e.g. signal planes) and 118A-118B (e.g., power planes) do not contact the conductive lining of the first PTH 110A and are therefore not connected to the first pin 107A. Likewise, the conductive layers 114A-114D (e.g. ground planes) and 116A-116B (e.g., signal planes) do not contact the conductive lining of the second PTH 110B and are therefore not connected to the second pin 107B.
As shown, the solder 140 only partially fills the openings of the PTHs 110A-110B. This may be due to the heat sinking effects caused by the ground or power planes 114A-114B and 118A-118B that are coupled to conductive linings of the PTHs 110A-110B. That is, during the soldering process, molten solder 140 fills the openings of the PTHs 110A-110B from the bottom to the top via capillary action, losing heat in the process. If the molten solder 140 cools too rapidly, it may freeze prematurely, causing the opening in the PTHs 110A-110B to be only partially filled as shown. Since the PTHs 110A-110B are coupled to potentially large sheets of copper (e.g., the ground or power planes 114A-114D, 118A-118B) which have a high heat transfer coefficient, the heat of the molten solder 140 is dissipated rapidly through these electrical and heat conducting layers.